SUNRPC: Fix a bug in call_decode()

call_verify() can, under certain circumstances, free the RPC slot. In that
case, our cached pointer 'req = task->tk_rqstp' is invalid. Bug was
introduced in commit 220bcc2a (SUNRPC:
Don't call xprt_release in call refresh).

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>